The present invention relates to zinc sulfide phosphor or luminescent material for use in electronic devices such as cathode ray tubes or the like and more particularly, to a material constituted by ceramic particles of zinc sulfide phosphor having flake-like shape or large particle diameter, and method of manufacture thereof.
Previously, it has been discovered by the present inventors that, when crystals of basic zinc sulfate are deposited by precipitation from a solution containing zinc sulfate and urea, the crystals take the flake-like shape as disclosed in U.S. patent application Ser. No. 864,447, now abandoned assigned to the same assignee as the present application. The present invention intends to provide ceramic particles of zinc sulfide phosphor having flake-like configuration or large particle diameter by using flake-like crystal particles of basic zinc sulfate obtained in the above described manner as raw material.
Conventionally, the luminescent screen of a cathode ray tube, for example, is constituted by application of phosphor particles of approximately spherical shape having particle diameter of 5 to 10 .mu.m onto a substrate such as a front glass of the cathode ray tube to the thickness of 10 to several tens .mu.m. Since the brightness or luminescence of a cathode ray tube largely depends on how the luminescent screen is formed, it becomes necessary to constitute the luminescent screen in such a manner that luminescence from the phosphor is taken out at the front of the luminescent screen as effectively as possible. For the above purpose, it is required to increase light transmittance of the fluorescent film through reduction of scattering and absorption of light within the fluorescent film, which is largely affected by the size and configuration of the phosphor particles. Especially, the configuration of the particles is very important, and when the light transmittance of the fluorescent film itself is improved by spreading the phosphor particles of flat flake-like shape onto the front glass, the luminescent efficiency of the luminescent screen is improved with a consequent high luminescence intensity. The present invention, when applied particularly to color, monochrome and projection type televisions, flat panel display, plasma display and the like, provides display equipment having a high display effect.
Known zinc sulfide phosphor material is prepared by adding activator to zinc sulfide sediments produced by passing hydrogen sulfide through an aqueous solution of zinc sulfate for subsequent calcination in hydrogen sulfide or together with flux. In the conventional method as described above, zinc sulfide obtained by the settling is in the form of fine powder with poor crystallization, and even after the subsequent calcination, the particle diameter thereof is only in the region of 5 to 10 .mu.m at most. Moreover, it is difficult to control the particle shape into flake-like configuration, etc., and from the viewpoint of mass productivity, it is considered to be almost impossible to produce phosphor of flake-like shape or of large particle diameter by the conventional methods.